Active-pixel image sensors are widely used in image devices, such as cameras, video recorders, and the like. Conventionally, active-pixel sensors are formed of charge-coupled devices (CCDs). As an alternative, active-pixel sensors may also be formed of photo diodes using complementary metal-oxide-semiconductor (CMOS) processes.
FIG. 1 illustrates a commonly known integrated active-pixel sensor, which includes an array of cells. FIG. 2 illustrates one of the cells in the array. The cell includes reset transistor M1, source follower M2, and select-gate transistor M3. Photo diode PD is coupled to the gate of source follower M2 and the source of reset transistor M1.
The operation of the cell is discussed as follows. First, a logic high signal is applied to the gate of reset transistor M1, so that photo diode PD is reverse-biased by power supply voltage VDD. A cross-sectional view of photo diode PD is schematically illustrated in FIG. 2, wherein photo diode PD includes an n+ region in contact with a p-substrate. Under the reverse bias by power supply voltage VDD, depletion region DR is generated. Accordingly, photo diode PD acts as a capacitor that is charged by power supply voltage VDD. Node NA is thus charged to a high voltage. Transistor M1 is then turned off and the charges are stored at node NA.
During the operation of photo diode PD, it may or may not be exposed to light. If photo diode PD is exposed to light, electron-hole pairs are generated in depletion region DR, and hence the charges stored at node NA are discharged to the ground by a current flowing through photo diode PD. Accordingly, the voltage at node NA is brought down. If, however, photo diode PD is not exposed to light, no electron-hole pairs are generated in depletion region DR, and the charges stored at node NA remain. Accordingly, the voltage at node NA is at least higher than if photo diode PD is exposed to the light. Such a voltage difference may be used to determine whether the respective cell is exposed to light.
The charges at node NA affect the operation of source follower M2, which acts as an amplifier amplifying the state at node NA without draining the charges at node NA. If photo diode PD is exposed to light, the voltage at node NB will be lower, otherwise, it will be higher. When line RS′ is set to logic high, select-gate transistor M3 is turned on to connect node NB to a column line. Accordingly, the voltage at the column line reflects whether photo diode PD is exposed to light or not.
The sensor array as shown in FIG. 1 suffers from drawbacks. The sensitivity of photo diode PD dominates the performance of the sensor array. However, the designers of the image sensor array face a dilemma. If the area of photo diode PD is smaller, the sensitivity of photo diode PD will be low. Conversely, if the area of photo diode PD is increased to improve the sensitivity, the response time of the sensor array will be sacrificed, and the resolution of the sensor array will be low.